There has been widespread shock after President Lech Kaczynski was killed along with many other senior Polish figures in a plane crash in western Russia early on Saturday.
Here is a selection of reactions from around the world.
POLISH PRIME MINISTER DONALD TUSK
We stand in the face of an incredible tragedy, one of the biggest tragic events of our nation's history. It is definitely the most tragic event in the post-war history of Poland.
Amongst the dead are many people than we knew personally, many friends. But it is their families who today suffer the most. On behalf of all Polish people I would like to express to them our condolences and sorrow. Specifically to the family of president Lech Kaczynski - his daughter, mother, brother and granddaughter.
RUSSIAN PM VLADIMIR PUTIN
Nothing like this has ever happened before.
RUSSIAN PRESIDENT DMITRY MEDVEDEV
These past days we together have been conducting commemorative events in Katyn, together have been mourning the victims of totalitarian times.
Lech Kaczynski was coming to Russia to personally pay tribute to the killed Polish officers, both as a president and a citizen of his country.
All Russians are sharing in your grief and mourning. I promise that all the circumstances of this tragedy will be investigated most thoroughly, in closest cooperation with the Polish side. I have given exhaustive instructions to the law-enforcement agencies.
On behalf of the Russian people, I am expressing my deepest, most sincere condolences to the people of Poland, sympathy and support for the victims' families and friends.
Monday, 12 April, will be declared a day of national mourning in Russia.
UN CHIEF BAN-KI MOON
Kaczynski served his country and people with distinction and conviction, and was deservedly respected internationally. That he died en route to an event marking a new level of reconciliation between Poland and Russia is particularly poignant.
QUEEN ELIZABETH II
It is with the deepest sorrow that I have learnt of the death of President Lech Kaczynski and First Lady Madame Kaczynska.
I recall President Kaczynski's long and distinguished public service and his role in the Solidarity movement. The deaths of many other of Poland's leading figures, including former president in exile Kaczorowski, only serve to deepen this tragedy.
I send my deepest sympathy to you and to the whole Polish nation.
UK PRIME MINISTER GORDON BROWN
I think the whole world will be saddened and in sorrow as a result of the tragic death in a plane crash of President Kaczynski and his wife Maria and the party that were with them.
We know the difficulties that Poland has gone through, the sacrifices that he himself made as part of the Solidarity movement. We know the contribution he made to the independence and the freedom of Poland.
I think at a time like this we also remember a family in mourning and large numbers of people who died and a whole country that will be shocked and saddened by what has happened.
US PRESIDENT BARACK OBAMA
Today, there are heavy hearts across America. The United States cherishes its deep and abiding bonds with the people of Poland.
It is a testament to the strength of the Polish people that those who were lost were travelling to commemorate a devastating massacre of World War II as the leaders of a strong, vibrant, and free Poland.
That strength will ensure that Poland emerges from the depths of this unthinkable tragedy, and that the legacy of the leaders who died today will be a light that continues to guide Poland - and the world - in the direction of human progress.
FRENCH PRESIDENT NICOLAS SARKOZY
Lech Kaczynski devoted his life to his country. A tireless defender of the ideas in which he believed, he always battled with conviction for the values that founded his entry into politics: democracy, liberty and the fight against totalitarianism.
GERMAN CHANCELLOR ANGELA MERKEL
This is a political and human tragedy for Poland, our neighbouring country. Lech Kaczynski was a real lobbyist for his country, he loved his country and he was an argumentative European.
I remember many meetings with him on European and international levels, in the end we always found a solution. I know that all of his life was dedicated to the fight for Poland's freedom and Europe's freedom.
My husband and I fondly remember (Kaczynski's) invitation to his country house near Gdansk where we spent many, many hours talking about Polish and European history.
Lech Kaczynski will be missed in Germany, as well. Today, Germany is grieving with the Polish people.
ITALIAN PRIME MINISTER SILVIO BERLUSCONI
It is a grave loss that has stricken a friendly country and in which I am sharing in with my heart.
PRESIDENT OF EUROPEAN PARLIAMENT JERZY BUZEK
It is difficult to comprehend the tragedy. It was difficult to imagine that a few dozen people, many of whom were elected by us, the Poles, in every part of our homeland, to be our representatives, would die within a few seconds while on duty, on duty for each of us.
It was a special and unusual trip. It was about the memory of the events 70 years ago. Now we have another great memorial and we will remember that memorial - also on Russian soil near Smolensk, near Katyn - on the date 10 April. This is a terrible coincidence, and let us be together, let us talk to each other.